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Monday, 13 January 2014

Bird Atlas 2007-11: what does it say about Swifts?

The preface to this monumental work says that this book will set the agenda for bird conservation for the next 20 years. We are therefore interested to see what it says about Swifts, in particular what has happened since the previous Atlases in 1968-72 and 1988-91?Perhaps the most important takeway is that Swifts have been declining for at least 40 years, something not brought out in more recent datasets.

Click maps to enlarge

The Breeding Distribution Map shows all those 10km squares in which at least 1 pair of Swifts was found possibly, probably or proved breeding. Swifts breed in virtually every square apart from north and west Scotland, and the western part of Ireland. If one compared it with the equivalent map for 1988-91, it would appear similar because relatively few squares have completely lost, or gained Swifts in the intervening years. However, the two maps below reveal more clearly what has actually been going on.

The Breeding Relative Abundance Change Map gives a graphical representation in the change in occupied tetrads in each 20km square between the two Atlases. Red signifies an increase, blue a decrease: the darker the colour the bigger the change.This map shows that rather more squares decreased in this measure than increased. Though the overall pattern is one of decline, there are some areas of good news, including southern Ireland in a country where overall there have been the most decreases.

The Breeding Distribution Change Map highlights those 10km squares that have completely lost or gained breeding Swifts since the previous two Atlases in 1968-72 and 1988-91. A black triangle means a loss, a red triangle means a gain. A small triangle signifies the change since 1968-72 and a large triangle since 1988-91. There are far more losses than gains. It is clear that Swifts have been losing ground for at least the last 40 years, As one might expect, the losses are concentrated in those areas where Swifts are more sparsely distributed, particularly Ireland, parts of Wales and north and western Scotland. A low density population is more likely to disappear than a higher one. It is on the edges of a species distribution where change is most obvious. So the Atlas is telling the same story as the Breeding Bird Census and the trends in BirdTrack reporting rates. It is helpful that there is no ambiguity in the trends determined by these three BTO datasets, it means conservation decisions can be made with there being little doubt about what the data is telling us.You can read more about the Atlas, with an excellent promotional video hereAcknowledgementsThank you to Dawn Balmer and the BTO for permission to publish these maps.ReferenceBalmer, D.E., Gillings, S., Caffrey, B. J., Swann, R. L., Downie, I. S. & Fuller, R. J. 2013. Bird Atlas 2007-11: The breeding and wintering birds of Britain and Ireland. BTO Books, Thetford.

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